1. Field of the Invention
The present invention relates, in the driving of a liquid crystal display device, to a processing method and a processing circuit for compensating image data in order to improve the response speed of the liquid crystal; more particularly, the invention relates to a processing method and a processing circuit for compensating the voltage level of a signal for displaying an image in accordance with the response speed characteristic of the liquid crystal display device and the amount of change in the image data.
2. Description of the Related Art
Liquid crystal panels are thin and lightweight, and their molecular orientation can be altered, thus changing their optical transmittance to enable gray-scale display of images, by the application of a driving voltage, so they are extensively used in television receivers, computer monitors, display units for portable information terminals, and so on. However, the liquid crystals used in liquid crystal panels have the disadvantage of being unable to handle rapidly changing images, because the transmittance varies according to a cumulative response effect. One known solution to this problem is to improve the response speed of the liquid crystal by applying a driving voltage higher than the normal liquid crystal driving voltage when the gray level of the image data changes.
For example, a video signal input to a liquid crystal display device may be sampled by an analog-to-digital converter, using a clock having a certain frequency, and converted to image data in a digital format, the image data being input to a comparator as image data of the current frame, and also being delayed in an image memory by an interval corresponding to one frame, then input to the comparator as image data of the previous frame. The comparator compares the image data of the current frame with the image data of the previous frame, and outputs a brightness change signal representing the difference in brightness between the image data of the two frames, together with the image data of the current frame, to a driving circuit. If the brightness value of a pixel has increased in the brightness change signal, the driving circuit drives the picture element on the liquid crystal panel by supplying a driving voltage higher than the normal liquid crystal driving voltage; if the brightness value has decreased, the driving circuit supplies a driving voltage lower than the normal liquid crystal driving voltage. When there is a change in brightness between the image data of the current frame and the image data of the previous frame, the response speed of the liquid crystal display element can be improved by varying the liquid crystal driving voltage by more than the normal amount in this way (see, for example, document 1 below).
Because the improvement of liquid crystal response speed described above involves delaying the image data in order to detect brightness changes by comparing the image data of the current frame with the image data of the previous frame, the image memory needs to be large enough to store one frame of image data. The number of pixels displayed on liquid crystal panels is increasing, due especially to increased screen size and higher definition in recent years, and the amount of image data per frame is increasing accordingly, so a need has arisen to increase the size of the image memory used for the delay; this increase in the size of the image memory raises the cost of the display device.
One known method of restraining the increase in the size of the image memory is to reduce the image memory size by allocating one address in the image memory to a plurality of pixels. For example, the size of the image memory can be reduced by decimating the image data, excluding every other pixel horizontally and vertically, so that one address in the image memory is allocated to four pixels; when pixel data are read from the image memory, the same image data as for the stored pixel are read repeatedly for the data of the excluded pixels, (see, for example, document 2 below).
Document 1: Japanese Patent No. 2616652 (pages 3-5, FIG. 1)
Document 2: Japanese Patent No. 3041951 (pages 2-4, FIG. 2)
A problem is that when the image data stored in the frame memory are reduced by a simple rule such as removing every other pixel vertically and horizontally, as in document 2 above, amounts of temporal change in the image data reconstructed by replacing the eliminated pixel data with adjacent pixel data may not be calculated correctly, in which case, since the amount of change used in compensation of the image data is erroneous, the compensation of the image data is not performed correctly, and the effectiveness with which the response speed of the liquid crystal display device is improved is reduced.
The present invention addresses this problem, with the object of enabling amounts of change in the image data to be detected accurately while requiring only a small amount of image memory to delay the image data, thereby enabling image data compensation to be performed accurately.